"Our friendship has lasted for a very long time."
Translation:Notre amitié dure depuis très longtemps.
When you use depuis like this, you have to use French present tense. This is because the action continues from the past, but carries on into the present time. The French past tenses are only allowed for completed actions, which took place totally in the past.
English is one of few languages that has a present perfect tense, which is used in this translation (has lasted). Most languages use the present tense, along with a duration word like depuis.